Method for producing steel slabs

ABSTRACT

The invention relates to a method for producing steel slabs, whereby the strand leaves an ingot mold with melt enclosed by a strand shell and, in a strand guide located downstream, the opening width of guide rolls mounted in segments with upper and lower frames can be set in a continuous manner with successive steps by adjusting elements that join said guide rolls. An oscillation around a center line of the opening width serves to modify the opening width in order to render the dynamic influences on the guide rolls negligibly small. The amplitude of the opening width oscillation is set to a quantity, which does not provoke any plastic deformation of the strand shell. The actual opening width of the guide rolls is detected and the actuating force of the adjusting elements as well as the amplitude of the actuating force are determined at the same time. In the case of an increasing amplitude of the actuating force, the opening width is set to a predeterminable measure and/or guided in a pressure controlled manner via at least one adjusting element. A force/path diagram of the opening width setting is then determined from a multitude of force/path measurements along the strand. Said diagram is composed, in a chain-like manner, of a multitude of small force/path curves, which are each provided in the form of a hysteresis and which can be individually determined using an oscillation with a comparatively small amplitude.

[0001] The invention pertains to a method for producing steel slabs,where the strand leaving the mold contains molten metal enclosed by astrand shell, and where, in a following strand guide system, the gapwidth of the guide rolls, which are supported in segments in an upperand a lower frame, can be adjusted continuously in successive steps byadjusting elements which connect the rolls, where

[0002] (a) the gap width is changed by oscillation around a centerlineof the gap width in such a way that the dynamic influences on the guiderolls are negligibly small;

[0003] (b) the amplitude of the gap width oscillations is set to a valuewhich does not cause any plastic deformation of the strand shell;

[0004] (c) the current gap width of the guide rolls is detected;

[0005] (d) simultaneously, the actuating force of the adjusting elementsand the amplitude of the actuating force are determined; and

[0006] (e) when the amplitude of the actuating force increases, the gapwidth is adjusted to a predetermined value and/or is guided as afunction of pressure by means of at least one adjusting element.

[0007] The method described in DE 197 20 768 C1 has the goal of creatinga basis on which it is possible to adjust the gap width with precisionover the entire strand guide system by the use of simple measuringdevices and simple mathematical means. It also has the goal of making itpossible to determine reliably the current position of the tip of thecrater inside the slab.

[0008] The method is based essentially on a dynamic measurement byexcitation of the strand in the form of oscillations, as a result ofwhich it can be recognized whether the gap width is tending to becomelarger or smaller than the optimum gap width at a specific measurementsite, so that, if a deviation is detected, suitable measures can betaken to correct it.

[0009] When the dynamic measurement is made, the actuating forceproduces a hysteresis curve relative to the gap width. This curve has arelatively small area when the shell is still thin and the craterrelatively large. The hysteresis curve has a relatively large area,however, after the shell has grown and the volume of the crater hasdecreased. The curve has a very slender form when the strand iscompletely solidified.

[0010] In the known method, the gap width is changed by an oscillationaround a predetermined centerline of the desired slab thickness.Characteristic force-distance curves are analyzed, for which purpose theslopes of the characteristic curves in the force-distance plane, theirpoints of intersection, etc., are taken into consideration for therequired evaluation.

[0011] Although it is stated in the document cited that the selectedoscillation value should be chosen so that the dynamic influences on thestrand shell, which is still relatively thin after leaving the mold,remain negligibly small and that the amplitude of the oscillating gapwidth should be set to a value which prevents the plastic deformation ofthe strand shell, the expert cannot derive any clear, reproducibleprinciple for concrete action from this information. For the knownmethod is based at least implicitly on the fact that the amplitude ofthe force-distance characteristic on which the mathematical analysis isbased is determined within the scope of only a single oscillationperiod, which means that the measurement distance consists only of thedistance covered by a single stroke.

[0012] Practical measurements in the situation described above, however,have shown that the stroke would have to be so large that, depending onthe starting point of the oscillation, there would be an acute danger ofthe plastic deformation of the strand shell, deformation which wouldlead to cracks in the microstructure.

[0013] Against the background of the insights and practical findingsdescribed above, the invention is based on the task of improving andelaborating the known method in such a way that the technicalmeasurements which are performed yield optimum results, so that thedisadvantages and difficulties caused by harmful deformation of thestrand shell or disturbances of its microstructure, such as cracking,can be avoided with all possible reliability.

[0014] To accomplish this task, it is proposed within the scope of amethod of the type indicated in the introductory clause of claim 1, thata diagram of the gap width settings be determined on the basis of aplurality of force-distance measurements along the strand, this diagrambeing composed of a plurality of small force/distance curves forming achain-like pattern, each curve being in the form of a hysteresis curve,each of which can be determined by means of an oscillation ofcomparatively small amplitude. Thus the disadvantages and difficultiesof the previous type of measurement procedure are reliably avoided, anda strand with the optimum microstructure can be obtained.

[0015] It is important in actual casting operations for the optimum gapwidth of the strand guide system to be determined and adjusted not onlyreliably but also as quickly as possible. For this reason, it shouldtherefore be possible to determine whether the gap width is “too big” or“too small” after each small-amplitude oscillation. For this purpose,the slope of each small hysteresis curve of the “chain” is evaluated.This corresponds to the stiffness of the “segment-strand” system at themeasurement site in question. This stiffness value can then be comparedwith values stored in a database, which-has been prepared for thesegment control system. This database contains empirical stiffnessvalues, which pertain to strands of different widths and solidificationstates such as those characterized by a liquid crater or the advancingsolidus/liquidus transition region.

[0016] A comparison with values of this database, which can beinterpolated or extrapolated as required, provides information on howthe actual gap width compares with the optimum gap width.

[0017] An elaboration of the invention also provides that the positionof the tip of the crater can be determined from a change in the area ofthe hysteresis curve, e.g., from a decrease in the area.

[0018] And, finally, it is advantageous that an increase in theconsistency of the strand can be determined from a change in the area ofthe hysteresis curve, such as from a decrease in the area.

[0019] An example of the invention is presented in the form of theattached drawing:

[0020] The FIGURE shows a force-distance diagram of the gap widthsetting. This consists of a plurality of small force-distance curves,each of which was determined individually by means of one oscillation ofsmall amplitude. On this basis, both a reliable and also a rapiddetermination and adjustment of the optimum gap width can be recognizedand implemented within the framework of practical casting operations. Inaddition, it is possible to know whether the gap width is too big or toosmall and must be corrected after only one oscillation of very smallamplitude. The individual curves, each representing a hysteresis curve,engage with each other like the links of a chain and show the course ofthe actuating force versus the gap width along the strand.

[0021] The invention therefore represents, so to speak, an “x-ray image”of the strand and provides an optimum solution to the problem describedabove.

1. Method for producing steel slabs, where the strand leaving the moldcontains molten metal enclosed by a strand shell, and where, in afollowing strand guide system, the gap width of the guide rolls, whichare supported in segments in an upper and a lower frame, can be adjustedcontinuously in successive steps by adjusting elements which connect therolls, where (a) the gap width is changed by oscillation around acenterline of the gap width in such a way that the dynamic influences onthe guide rolls are negligibly small; (b) the amplitude of the gap widthoscillations is set to a value which does not cause any plasticdeformation of the strand shell; (c) the current gap width of the guiderolls is detected; (d) simultaneously, the actuating force of theadjusting elements and the amplitude of the actuating force aredetermined; and (e) when the amplitude of the actuating force increases,the gap width is adjusted to a predetermined value and/or is guided as afunction of pressure by means of at least one adjusting element,characterized in that a force/distance diagram of the gap width settingis determined on the basis of a plurality of force/distance measurementsalong the strand, which diagram is composed of a plurality of smallforce/distance curves linked like a chain, each in the form of ahysteresis curve, each of which can be determined by means of anoscillation of comparatively small amplitude.
 2. Method according toclaim 1, characterized in that each curve, determined in the form of ahysteresis curve, is determined and evaluated in accordance with itsslope as an index of the current stiffness of the “segment-strand”system.
 3. Method according to claim 1 or claim 2, characterized in thatthe characteristic stiffness value which has been determined is comparedwith data stored in a database supplied with empirical values, which canbe interpolated or extrapolated as required, and in that recognizabledeviations are corrected by appropriate adjustment of the gap widths. 4.Method according to one or more of claims 1-3, characterized in thatwhether the adjusted gap width of the guide rolls is deviating from theoptimum value in the direction of “too big” or “too small” is determinedon the basis of each hysteresis curve after only one oscillation ofcomparatively small amplitude, whereupon a corresponding correction ismade.
 5. Method according to one or more of claims 1-4, characterized inthat the position of the tip of the crater is determined from a changein the area of the hysteresis curve, e.g., in the case of a decrease inthe area.
 6. Method according to one or more of claims 1-4,characterized in that an increase in the consistency of the strand isdetermined from a change in the area of the hysteresis curve, e.g., inthe case of a decrease in the area.